Since the appearance of semiconductor devices, one of themes for which technical efforts have been most focused on semiconductor devices is to increase the operating frequency (clock frequency). As the operable frequency of transistors has increased to enable high-speed operations, the application range thereof has also expanded so that the transistors have contributed to the realization of the current broadband-network society and to the IT development. The operating frequency of semiconductor devices has been increased year by year to reach the range of GHz, while, in recent years, the speed of the improvement has been slowed down. In the current state, the operating frequency is staying at 3 to 4 GHz and there is a demand for early realization of a semiconductor device that can operate at a clock frequency of 10 GHz or more. One of factors related to the increase in operating frequency of a semiconductor device is the size. As the size decreases, the capacitance decreases and thus the transmission speed of signals (the signal transmission speed) increases.
As is well known, in recent years, the speed of miniaturization of semiconductor devices has been slowed down, which has been a factor of impeding the improvement in operating frequency. Assuming that the size of a semiconductor element is reduced to its limit and is constant, a factor for increasing the operating frequency is an operating current of the element. With a MOSFET as an example, one of factors that determine the operating frequency is how much drain current is allowed to flow depending on a gate voltage when the drain voltage is set constant. As the drain current increases, the charge/discharge time of the output capacitance decreases and thus the signal transmission speed to the next stage increases. However, if the size of the element is reduced to its limit, it is difficult to increase the drain current in the present state of art, which impedes the improvement in operating frequency.
As described above, even if it is attempted to set the operating frequency of a semiconductor device to, for example, about 10 GHz or more, it is difficult to increase the drain current in the case of a transistor reduced in size to its limit. In view of this, it has hitherto been considered impossible to realize a high-speed semiconductor device that can operate at about 10 GHz or more, preferably at about 20 GHz or more, and further even at 40 to 60 GHz.